The present invention relates to the electrochemical synthesis of polyols, and more particularly, to improved methods for the electrochemical conversion of formaldehyde-containing electrolytes to alkylene glycols, such as ethylene glycol, propylene glycol, and the like.
Polyols, and in particular alkylene glycols are major industrial chemicals. The annual production rate of ethylene glycol, for example, in the United States alone is about 4 billion pounds per year. Ethylene glycol is widely used as an automotive coolant and antifreeze. It also finds major applications in manufacturing processes, such as in the production of polyester fibers. In addition to such major uses as heat transfer agents and fiber manufacturing, alkylene glycols also find use in the production of alkyd resins and in solvent systems for paints, varnishes and stains, to name but a few.
The major source of ethylene glycol is derived from the direct oxidation of ethylene from petroleum followed by hydration to form the glycol. However, dwindling petroleum reserves and petroleum feedstocks coupled with escalating prices has led to the development of alternative routes for making polyols. For example, processes based on catalytic conversion of synthesis gas at high pressure appear to offer promise. The reaction for making ethylene glycol by this route may be shown as: EQU 2 CO+3H.sub.2 .fwdarw.HOCH.sub.2 --CH.sub.2 OH
Representative processes are described in U.S. Pat. Nos. 3,952,039 and 3,957,857.
Other attempts to produce ethylene glycol and higher polyols from non-petroleum feedstocks have involved the electrochemical route. Heretofore, electrochemical methods of organics manufacture have not been widely accepted mainly because they were generally viewed as being economically unattractive.
Tomilov and coworkers were apparently the first to reduce formaldehyde electrochemically in aqueous solution to ethylene glycol. This work was published in J. Obschei Khimii, 43, No. 12, 2792 (1973); Chemical Abstracts 80, 77520d (1974). Further work by Watanabe and Saito, Toyo Soda Kenkyu Hokoku, 24, 98 (1979); Chemical Abstracts, 93, 227381u (1980), aspects of which are described in U.S. Pat. No. 4,270,992 disclose the reduction of formaldehyde under alkaline conditions forming ethylene glycol at maximum current efficiences of up to 83%, along with small amounts of propylene glycol. However, most conversion efficiencies reported by Watanabe et al supra were not at such high levels although conducted under alkaline conditions.
More specifically, U.S. Pat. No. 4,270,992 discloses a method for making ethylene glycol or propylene glycol through electrochemical coupling of formaldehyde solution employing an electrochemical cell equipped with graphite electrodes. The U.S. patent provides that ethylene glycol is not formed under acid conditions, but instead a pH of more than 8 is required. Watanabe et al supra even tested various supporting electrolytes, including tetraethylammonium tosylate in a formaldehyde electrolyte under acid conditions without controlling the pH which resulted in low current efficiencies (26%).
U.S. Pat. No. 3,899,401 (Nohe et al) relates to the electrochemical production of pinacols like tetramethylene glycol from carbonyl compounds, such as acetone which may be converted into pinacolone or 2,3-dimethylbutadiene. Nohe et al do not teach the electrosynthesis of either ethylene or propylene glycol, but do mention one aldehyde, namely acetaldehyde which may be electrochemically reduced in an undivided cell. Like Watanabe et al supra, Nohe et al also mention quanternary ammonium salts. However, Nohe et al also require that such electrochemical reactions be conducted by the addition of up to 90 percent by weight alcohol, (for example, ethanol in the case of acetaldehyde reduction) to the electrolyte. By comparison, Weinberg and Chum, Abstracts of the Electrochemical Society Meeting, Abstracts No. 589, pages 948-949, May, 1982 reported that the presence of alcohol (methanol) in the electrolyte depresses the conversion efficiency of formaldehyde to ethylene glycol, and that the best conversion efficiencies were achieved with the lowest level of alcohol in the electrolyte.
The early studies by Tomilov et al supra related to the electrochemical reduction of formaldehyde under acid conditions i.e. pH from 2 to 5 using a graphite electrode in a medium of potassium dihydrogen phosphate solution and mercury (II) catalyst to form ethylene glycol at a current efficiency of 24.9%. The yields of glycols calculated on the aldehydes taken were 46.2 and 70.7%.
Accordingly, there is a need for a more reliable and efficient alternative for making alkylene glycols from non-petroleum feedstocks, and more particularly, there is a need for an improved electrochemical means for making ethylene glycol by the reduction of formaldehyde. By necessity, the electrochemical route should offer a high degree of product selectivity providing reproduceable results with more consistent, higher yields and current efficiencies to minimize electrical energy requirements. Correspondingly, such glycols should be formed at high concentrations for lower separation costs. Most optimally, the electrochemical condensation of formaldehyde in making ethylene glycol should provide for useful anode reactions utilizing electrolyte additives and cell components e.g. electrodes which will perform as electrocatalysts for optimum conversion of organic molecules to the desired end product.
The present invention provides such improved methods and apparatus for the electrosynthesis of lower alkylene glycols from non-petroleum based feedstocks, namely coal and biomass. More particularly, the invention disclosed herein relates mainly to the preparation of ethylene glycol, and other lower polyols with reduced levels of by-products through the electrochemical reduction of formaldehyde under conditions which make such routes economically feasible, and therefore, competitive with alternative chemical routes. The electrochemical reduction of formaldehyde can now be carried out at high current efficiencies by controlling both reaction conditions and electrolyte composition. The present invention also relates to improved electrochemical cell components which enhance the efficient conversion of formaldehyde to ethylene glycol and hence make the economics more attractive.